Floor warming... 120V mat vs. 240V thermostat

Discussion in 'Electrical Forum discussion & Blog' started by Fistor, Sep 30, 2009.

  1. Fistor

    Fistor Geotechnical Engineer

    Messages:
    48
    Location:
    Vancouver, BC
    Hi all -

    This is probably an obvious question, but my bathroom is being renovated, and the mat (Honeywell 10-15B-120H) is rated for 120V. A separate 20A circuit was installed for this.

    The thermostat that was purchased is a Honeywell also (Honeywell HWF1GB-15-240), rated for 240V.

    Does the thermostat also need to be rated 120V, in order to be used with the 120V mat?
    (sorry if this is obvious, but I want some opinions before I buy a 120V).

    Thanks!
  2. PEW

    PEW DIY Senior Member

    Messages:
    487
    You can use a 240v rated thermostat on a 120v mat. However, would strongly suggest you use a floor sensing thermostat not an air sensing one. You will get a considerably better economy and comfort.
  3. Fistor

    Fistor Geotechnical Engineer

    Messages:
    48
    Location:
    Vancouver, BC
    Hey, thnx Pew, I was hoping someone would say that... not sure why I got the 240-V, but I had thought that since the load was in the mat, maybe my mistake was recoverable...

    Oh, and yes, I do have the floor sensing one - I already installed the thermostat lead with the mat.

    Thnx again!

    Note after first reply: further checking on the 'net turns up comments such as this one: "Must be used with 240 volt floor heating system only", which I found on Home Depot... I assume that this is to avoid confusion and the use of a 240V mat with a 120V thermostat....? any thoughts? Is there anything risky or potentially damaging about using a 240V controller on a 120V mat?
    Last edited: Sep 30, 2009
  4. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Generally, the difference between a 120 volt and 240 volt line voltage thermostat is the number of wires. A 120 has one line in...one out. The 240 would have 2 in and 2 out. Seems like no downside to just using one set of in/out. A quick check with an ohmeter could tell you which wires are a set.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,154
    Location:
    New England
    The contacts in the thermostat might not handle the current. Remember, that when you halve the voltage, the current doubles to maintain the same number of watts. So, if the internal switches are designed for that lower current at 240vac, you'd burn it up at 120 since for the same number of watts, you need twice the current. Also, depending on how it is configured, the internal power supply may not provide the required voltages for the display. This is a question you should ask of Honeywell, or peruse the spec sheet carefully. For a general rule, you can always use a mechanical switch designed for higher voltages on a system with lower voltages. With an electronic device, the input voltages must be within the design specifications. Assuming this thing is electronic and gets line voltage, unless the specifications say 110, a 240v version won't work. Simple non-electronic switch, possibly.
  6. leejosepho

    leejosepho DIY scratch-pad engineer

    Yes, that was my first thought. The internals are expecting 240v that will likely be transformed to whatever else for control.

    Also, I use an Aube AF (air-floor) thermostat that has a floor sensor for high limit, then an air sensor to turn it on ... and that works great. Since your floor temp is always higher than room temp, not having an air sensor could result in a hot air temp in the room being fed even more by a floor that does not know the room is already too warm. An AF thermostat will keep the floor where you want it without letting it add excessive heat to the room (like if somebody had the sun lamp on for a while and the next person would like the room to cool a bit before washing up for dinner).
  7. Fistor

    Fistor Geotechnical Engineer

    Messages:
    48
    Location:
    Vancouver, BC
    Excellent replies guys, they were the kind of feedback I need....

    That was the one thing I was really worried about - whether or not the electronic controls (and they are) could handle the difference.

    I agree with your comment about current draw, but it looks like the specs say that both the 120V version and the 240V version (mine) draw a max. current of 15A, with the difference being the power rating: 1800W for the 120V, and 3600W for the 240V controls. If it was only that, then I'd not be as concerned (I think...?)

    However, the "proportional integral adaptive" controller (that's what they call it) they say is different than a regular electromechanical thermostat... apparently, to maintain the setpoint temperature, the controller looks at the amount of power required by the mat, apparently by percentage... it has 5 settings (0-20%, 20-40%, etc.). I wonder if it will be unable to achieve a higher setting, as maybe the most that the mat could generate would be 50% (i.e. 1800W, as opposed to the max. 3600W that the thermostat might expect).

    My understanding about these systems is that the heating zone is very limited in distance from the floor - the system heats the floor tiles only, and maybe a zone that extends only a few millimeters above the floor... as opposed to a radiant floor system, which is meant to heat the room as well... anyone know if this understanding is correct?

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The mat is already installed, so all that's left is to connect the controller... I wonder if a test period would be sufficient to judge the effectiveness... or is there some hazard that I haven't considered...?

    (I wouldn't expect a hazard, unless my mat tried to use more power than the thermostat could handle, but this is the opposite situation...)

    Any additonal thoughts?
    Last edited: Sep 30, 2009
  8. leejosepho

    leejosepho DIY scratch-pad engineer

    As far as I know, you are correct in believing your wire is not intended for heating a room. Nevertheless, the heat it produces does have to radiate to somewhere, and that would be into the room. I do not have central air, and I do not want my bathroom floor to match an ambient temp of 80* in the summertime. So, the air temp in the room decides whether or not the floor gets heated at all, then the floor sensor keeps that floor temp limited.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,154
    Location:
    New England
    Unless the spec sheet says it will work on 120vac, do NOT use the electronic device on 120. Get a 120vac version. If it were purely mechanical, it would probably work.

    Note, you could probably swap the breaker and run it as 240vac, if you have another slot in the panel. I'm not positive, but expect the heating wires are the same...you'd need to verify that. SOmetimes you have a lot of trouble exchanging electronic devices...a new CB may be the cheapest. This assumes this is on its own, dedicated circuit, or could be made to be dedicated.
    Last edited: Oct 1, 2009
  10. leejosepho

    leejosepho DIY scratch-pad engineer

    I hope I am not confusing anything here, but I think I see a potential fire!

    The thermostat does not "draw" 1800 or 3600 watts, it can simply handle that amount. The heating wire is what draws current, and a 110v-rated wire can definitely *not* handle 240 volts. I almost learned that the hard way with my own wire, but someone who has since been banned from here saved my house by catching my error beforehand. Read the resistance of your wire and do the math (ohms law) at probably 12 watts per square foot of floor coverage and you will know how much voltage it can stand!
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,154
    Location:
    New England
    The controller might not send line voltage to the heating wires, if it does, then the wires would be different based on the expected input. There are several ways to control the current...adjust the voltage, or adjust the duty cycle.
  12. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    t'stat

    With 120 volts you will only use one set of contacts on the thermostat, you do NOT switch the neutal wire. If the t'stat has electronics it will NOT work since it will be looking for power across both sets of contacts, and thus will not have ANY power source. If you improperly connect the neutral wire, it WILL have a power source, but in that case it will be the wrong voltage, which will create its own problems, but at least not burn the t'stat out.
  13. Fistor

    Fistor Geotechnical Engineer

    Messages:
    48
    Location:
    Vancouver, BC
    Well, there was enough confusion on my part (and some in the replies) that I went ahead and ordered a replacement thermostat - I probably would have anyway, but I wanted the feedback.

    Yeah, I agree leejo, although if the mat is 120V, then that would be the "limiting factor", would it not? Anyway, I won't take any chances.

    On a side note, the company I ordered a replacement thermostat from is fantastic - not the one I originally ordered from, but the are rushing me a replacement, and set me up as a "contractor" so that I can return the other one and receive a refund - I am very impressed with their great service and attitude (I was pretty much prepared to own the 240V and let it sit in a box forever).

    Thanks again everyone for the replies!

    carl
  14. leejosepho

    leejosepho DIY scratch-pad engineer

    Yes. 120v X the ohm rating of your wire = its maximum, and 240v would overload it to 200% of its capacity. I initially made that kind of mistake and had to pay a restocking charge to return two 240v stats and get 120s.
Similar Threads: Floor warming
Forum Title Date
Electrical Forum discussion & Blog Two 12-4 Romex cables through the same floor joist holes Jul 5, 2014
Electrical Forum discussion & Blog How much romex in 2" PVC conduit from 1st floor to 2nd floor? Jun 13, 2013
Electrical Forum discussion & Blog Testing a Soun Touch floor heat mat. Mar 11, 2012
Electrical Forum discussion & Blog easy heat floor heating system Oct 26, 2011
Electrical Forum discussion & Blog Electric Floor heating cable Mar 3, 2011

Share This Page